1. Field of the Invention
The present invention relates to a motion state estimation apparatus for a vehicle adapted to estimate a motion state of a vehicle such as the state of acceleration in the fore-aft direction of the vehicle body and the state of turning of the vehicle.
2. Description of the Related Art
Conventionally, there have been widely known vehicle motion control apparatuses which perform various braking force controls such as antiskid control which at least decreases and increases brake hydraulic pressures supplied to wheel cylinders of wheels of a vehicle so as to adjust braking forces acting on the wheels, to thereby prevent the wheels from generating excessive slippage, and braking force distribution control which imparts a difference between braking force acting on the left-hand wheels of a vehicle and that acting on the right-hand wheels of the vehicle such that a predetermined yawing moment is generated in the vehicle.
In general, such a vehicle motion control apparatus estimates a motion state of a vehicle (for example, the state of acceleration in the fore-aft direction of the vehicle body and the state of turning) on the basis of wheel speeds of individual wheels obtained from wheel speed sensors, and starts and performs the above-mentioned various braking force controls on the basis of the estimated motion state (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-193085). However, when the motion state of a vehicle is estimated on the basis of wheel speeds, the estimated motion state of the vehicle involves an error, possibly resulting in a failure to achieve proper braking force control, as described below.
For example, when the vehicle motion control apparatus performs antiskid control, the apparatus sets an estimated vehicle body speed on the basis of the highest speed (hereinafter may be referred to as the “highest wheel speed”) among the wheel speeds of the wheels of the vehicle, and starts the antiskid control at least when the difference between the estimated vehicle body speed and the wheel speed (that is, the degree of slippage) becomes equal to or higher than a predetermined level. As a result, when the antiskid control is performed while the vehicle is traveling on an ordinary road surface such as paved road surface, the deceleration of the vehicle is maintained at a high level.
Meanwhile, when a wheel has generated an excess degree of slippage because of a driver's braking operation during a period during which the vehicle has been traveling on a road surface having a low-friction coefficient (hereinafter may be referred to “low-μ road surface”) such as an icy road surface, in general, all the wheels tend to start slipping to a generally equal extent. This trend easily occurs in particular in a four-wheel drive (4WD) vehicle in which all the wheels are connected with one another via a drive system.
When all the wheels start to slip to a generally equal extent as described above, a difference is produced between the highest wheel speed and the actual vehicle body speed, and thus an error is generated in the estimated vehicle body speed. As a result, a difference becomes less likely to be generated between the estimated vehicle body speed and the wheel speed, whereby the antiskid control becomes less likely to be started.
Moreover, in such a case, even when the antiskid control is started, due to a small rotational moment in a speed-increasing direction which each tire receives from the road surface, a relatively long time is required for the rotational speed of each wheel having once dropped to recover to a speed corresponding to the vehicle body speed as a result of control for lowering the brake hydraulic pressure. Accordingly, in this case, in order to secure traveling stability of the vehicle, restoring the rotational speed of the wheel must precede maintenance of the large deceleration.
In view of the above, the control mode (for example, control start conditions, brake hydraulic pressure decrease-increase pattern, etc.) of the antiskid control is desirably changed depending on whether the vehicle is traveling on a low-μ road surface such as icy road surface or an ordinary road surface such as a paved road surface. Such operation requires road surface determination; i.e., determination as to whether the road surface on which the vehicle is traveling is the above-mentioned low-μ road surface or ordinary road surface.
The acceleration (deceleration) of the vehicle body during a period during which the wheels undergo slippage because of driver's braking operation depends on the friction coefficient of the road surface. Accordingly, an estimated vehicle body acceleration (estimated vehicle body deceleration) can be calculated from a change in the estimated vehicle body speed, and the road surface can be determined on the basis of the calculated estimated vehicle body deceleration.
However, as described above, when all the wheels start to slip to a generally equal extent during a period during which the vehicle is traveling on a low-μ road surface, the estimated vehicle body speed involves an error. Therefore, the estimated vehicle body deceleration calculated from the estimated vehicle body speed differs from the actual vehicle body deceleration. As a result, in some cases the road surface determination may be performed erroneously. In other words, when the vehicle body deceleration (that is, the state of acceleration in the fore-aft direction of the vehicle body) is estimated on the basis of the wheel speeds, the estimated state of acceleration in the fore-aft direction of the vehicle body involves an error, sometimes resulting in failure to perform proper antiskid control.
Meanwhile, when the vehicle motion control apparatus performs the above-described braking force distribution control, the turning direction of the vehicle must be determined. When the vehicle turns, the wheel speeds of wheels located inside a turning locus become lower than those of wheels located outside the turning locus. On the basis of this phenomenon, the turning direction of the vehicle can be determined. Specifically, the turning direction of the vehicle can be estimated on the basis of the relation of the magnitudes of the wheel speeds of the left-hand and right-hand wheels.
However, in the case where one or more of the wheels separate apart from the road surface because of irregularities on the road surface, the relation of the magnitudes of the wheel speeds of the left-hand and right-hand wheels may become contrary to the relation corresponding to the turning direction. As a result, in some cases the turning direction is determined erroneously. In other words, when the turning direction of the vehicle (that is, the state of turning) is estimated on the basis of the wheel speeds, the estimated state of turning involves an error, sometimes resulting in failure to perform proper braking force distribution control.
As described above, when a motion state of a vehicle is estimated on the basis of the wheel speeds, in some cases the estimated motion state of the vehicle involves an error. Accordingly, in order to properly perform braking force control such as antiskid control or braking force distribution control, accurate estimation of a motion state of a vehicle, such as the state of acceleration in the fore-aft direction of the vehicle body or the state of turning of the vehicle, is desired.